The present invention relates to a vitreous china, a method for the preparation of the vitreous china, a sanitary-ware produced from the vitreous china and a glaze for the vitreous china.
The term "vitreous china" means a material which has a compact and uniform structure and which is fired till almost or substantially all of the open cells disappear and the term "vitrification" means the phenomenon that particles constituting the basis material are melted into a glass phase during firing and the glass phase is fluidized at a high temperature to thus fill up the gaps formed between the unmelted particles.
Whether a particular particle can be melted or not is dependent upon the melting point thereof which is influenced by the crystalline structure of the particle, combinations of the particular particle with other particles (a specific crystal which has a high melting point in itself may be melted at a temperature lower than the melting point thereof if other crystals coexist) and the firing temperature and time. In case of conventional vitreous china, raw materials capable of being melted during the vitrification phenomenon are, for instance, feldspars serving as fluxes end clays capable of imparting plasticity during molding, while raw materials which are not melted during the vitrification phenomenon are, for instance, quartz. However, quartz is not completely unmelted. More specifically, a part thereof is melted into the glass phase while the remaining part thereof holds its crystalline form.
Such a vitreous china has presently been used for the production of, for instance, sanitary-wares. Among the ceramic whiteware products, the sanitary-wares have relatively large sizes and complicated shapes and accordingly, have thick-walled structures for ensuring the mechanical strength thereof. The conventional vitreous china has a flexural strength ranging from 400 to 800 kgf/cm.sup.2 and products having such a flexural strength requires the wall thickness on the order of 7 to 12 mm.
The reason why quartz has been used as a crystalline phase-forming substance in the conventional vitreous china described above is that it is cheaper than other sources for the crystalline phase and that it is a naturally occurring raw material which coexists with glass phase-forming minerals. For instance, feldspars serving as fluxes and clays capable of imparting plasticity to the basis material during molding rarely independently occur naturally and in most cases, they occur in the form of admixtures with quartz. In addition, there has been known pottery stones which comprise all of these quartz, feldspars and clays and some of them per se can be used alone as a raw material for ceramic whitewares.
For this reason, minerals conventionally used as the crystalline phase-forming raw materials for ceramic whitewares are limited to quartz or cristobalite which is a crystal formed through transformation of quartz at a high temperature. As has been discussed above, however, a part of quartz is melted into the glass phase. Thus, it is very difficult to control the particle size of the quartz present in the resulting fired product and quartz used is often completely melted into the glass phase, in particular, when quartz as a starting material is used in the form of fine particles.
In the field of porcelain, there has recently been conducted studies to substitute other crystals for quartz for eliminating the limit in the particle size distribution encountered when such quartz is used as a crystalline phase and for eliminating the drawbacks originated from the difference between the thermal expansion coefficients of the glass phase and quartz (among various kinds of crystals, quartz is assigned to a group comprising crystals having very high thermal expansion coefficients). They are called, for instance, "alumina porcelain" in which alumina is substituted for quartz in the usual porcelain and "zircon porcelain" in which zircon is substituted for quartz in the usual porcelain, in distinction from the usual quartz-clay-feldspar porcelain. In this respect, the alumina porcelain is developed for mainly improving the strength of the porcelain, while the zircon porcelain is developed for mainly improving the electrical properties of the porcelain. The alumina porcelain has already been practically used in tablewares and insulators and examples thereof are disclosed in, for instance, Japanese Un-examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 50-6608.
As has been discussed above, the sanitary-ware formed from a vitreous china has a thick-walled structure because of its low strength. Incidentally, consumers have recently requested high-grade articles and, as a result, there has been a great demand for a sanitary-ware having a sharp shape (such as those having sharp-edged corners). However, thick-walled products cannot respond to this requirement. Moreover, the sanitary-ware in general has a large size and is installed within a narrow space. Therefore, the thick-walled product is heavy and this makes the installation thereof difficult.
In addition, the vitreous china which has been used in the production of sanitary-wares has low strength and thus cannot be used for producing other articles such as various kinds of engineering ceramics and large-sized ceramic plates.
Many attempts have been made, to eliminate the foregoing drawbacks, by substituting other crystalline substances as crystal phase-forming raw materials such as alumina for quartz as in case of porcelain, but such substitution is insufficient in case of vitreous china and suffers from various related problems to be solved.
For instance, high resistance to thermal shock is required for large-sized articles having complicated shapes such as sanitary-wares. The term "thermal shock" herein used includes that encountered during firing and that applied to articles after firing.
An article is liable to receive the thermal shock during firing in particular in the temperature-dropping stage of the process for firing the article in a furnace. More specifically, the temperature of the surface area of the article is lowered while that of the inner part of the article is still high during the temperature-dropping stage and this results in a temperature difference and hence the thermal shock of this kind. On the other hand, an article receives the thermal shock after firing, for instance, when hot water is poured into the article such as a wash-bowl.
Articles other than sanitary-wares such as tablewares of porcelain seem to also receive the thermal shock of this kind (tablewares are likewise subjected to temperature-dropping stage in a furnace during production and sometimes receive hot cooked foods), but this problem becomes more and more serious for large-sized, thick-walled articles having complicated shapes such as sanitary-wares. In other words, a large temperature difference is established between portions of a large-sized thick-walled article and this becomes a cause of thermal shock which results in a breakage of the article. A thick-walled article has resistance to breakage due to a difference in thermal expansion coefficients or a temperature difference weaker than that observed on a thin-walled article even if it is assumed that these thick-walled and thin-walled articles have the same temperature difference between the surface and the back face. Moreover, articles such as sanitary-wares comprise hollow portions therein and the air present in the hollow portions is not easily cooled. This results in application of substantially high thermal shock to the article during the temperature-dropping stage in a furnace.
Furthermore, the vitreous china comprises relatively coarse particles as compared with a porcelain and accordingly, is in general fired at a low temperature. For this reason, if other crystals having high melting points such as alumina are substituted for quartz having a low melting point, the particle size and composition of each raw material must be in accord with those of alumina.
In a slip casting method used as a means for molding, for instance, sanitary-wares, the properties of the slurry used for the molding must be controlled so as to make them favorable for the slip casting and this in turn requires strict control of the particle size and composition of each raw material and strict management of the method for producing a slurry.
Moreover, the surface of the sanitary-ware must be coated with a glaze and accordingly, the glaze must be well-compatible with the properties of the body made of vitreous china. This problem of compatibility is particularly important for large-sized articles having complicated shapes such as sanitary-wares because of a strong force acting on the interface between the body and the glaze.
As has been discussed above, there has not yet been developed any method for improving the strength of the vitreous china without accompanying various problems concerning production techniques and required properties of articles.